This invention relates to aircraft speed command systems and, more particularly, to a system for displaying to the pilot an angle of attach (alpha) error indication to enable him to maintain a pre-selected airspeed during takeoff.
One of the typical instruments on a commercial aircraft instrument panel is an attitude director indicator which has, among other things, a slow-fast indicator to indicate the aircraft speed with respect to some reference speed, and attitude director indicator command bar to indicate aircraft pitch command or angle of attack (alpha) information.
During takeoff, the aircraft speed is a function of pitch and angle of attack. That is, as the aircraft increases in pitch attitude, the aircraft speed will decrease. However, the aircraft speed must be maintained above the stall speed, and to guarantee a degree of safety, the attitude director indicator is preset to indicate to the pilot two visual indications relative to a reference speed which is a fixed percentage above stall speed or relative to a reference alpha.
One of these indicators is an attitude director indicator command bar, a horizontal bar which rises above or falls below a fixed line to indicate whether the aircraft angle of attack is respectively below or above the angle which would result in the aircraft just attaining the reference speed. The other indicator is the slow-fast indicator, a circular indicator which rises or falls against a fixed background and displays whether the aircraft speed is above or below the reference speed.
Thus, the indicator bar is a visual indication of the angular command for the aircraft while the slow-fast indicator is a visual indication of the speed of the aircraft.
In reaction to these speed and attitude indications, the pilot can push or pull the control wheel which changes the angle of the elevator in the tail assembly. This results in raising or lowering the nose of the aircraft and, thereby, decreasing or increasing the aircraft speed. In this way the pilot corrects for, or flies to, the attitude director indicators.
In a prior speed command system for use in commercial aircraft during landing, inputs to the system such as the actual angle of attack (alpha), a reference angle of attack (alpha reference), and an air speed error equaling the difference between actual and reference speeds are used as inputs to a computer, the output of which is the command bar or alpha error signal to the pilot as an indication for him to increase or decrease the aircraft angle of attack and, indirectly, the speed. The reference angle of attack is typically a maximum angle of attack which corresponds to the reference air speed which is a constant percentage above stall speed, and the output of the computer is a slow-fast indication to the pilot of the difference between the actual and reference alphas.
A second input to the speed command computer is an alpha error signal computed from air speed. For a given aircraft at typical landing speeds and loads, the air speed is a function of the angle of attack. Thus, if the difference between the minimum air speed and the actual air speed is multiplied by a constant, the results will be another alpha error signal.
Thus, two alpha error signals are generated; one from the difference between the angle of attack and the reference angle of attack, the other computed from the difference between the actual air speed and the reference air speed.
During landing, both of these inputs must be monitored. That is to say, if alpha becomes too great or if air speed is too low, an indication to the pilot to decrease the angle of attack will be issued by the speed control system. This is accomplished by monitoring both alpha error signals and generating a speed correction indication from the one which shows the worse case alpha error signal. In this way the speed command computer is continuously reacting to the more critical of the two error signals.
In adopting this prior system for the takeoff mode, a complication arises in that the error signal generated from air speed is not always correct. If the angle of attack is decreased, for instance, the difference between alpha and alpha reference will be immediately increased, but the air speed will be affected only gradually as the speed of the aircraft slowly increases in reaction to the decreased angle of attack. Thus, the error signal computed from the alpha angles leads the alpha error signal computed from air speed by a certain time constant. This transient error increases the amount of manual intervention required by the pilot and is therefore undesirable. This is especially true for use during takeoffs since the speed command indicator is constantly being monitored whereas in landing, for throttle control, it is intermittently monitored.
A second problem associated with this system for use during a takeoff is that aircraft frequently take off at speeds considerably in excess of the minimum speed upon which the reference angle of attack is based. This is necessary to allow the aircraft to take off with heavier loads or to allow the aircraft to take off at a speed which will allow it to maximize the altitude rate. In either of these cases the pilot needs a speed command other than one based on a reference speed at a predetermined percentage above stall speed, and the speed command system described above does not provide this type of indication.
Further, a takeoff flight path may require a variable airspeed, but the above-described speed control system is limited to displaying an error based on a fixed reference speed.